The invention relates to a process and a device for sputter coating a plastic substrate, preferably a polymethylmethacrylate substrate, with metal, particularly aluminum, by means of a direct current source. This direct current source is connected to an electrode disposed in an evacuable coating chamber and the electrode in turn is connected to a target which is sputtered and the sputtered particles are deposited on the substrate. A process gas is introduced into the coating chamber which can be connected via a gas supply line to a cylinder filled with argon and to a cylinder filled with helium. Valves for the metered supply of the gases to the coating chambers are interposed in the gas supply lines.
In known processes, an aluminum layer is directly sputtered onto a plastic substrate, e.g. on polycarbonate, without an intermediate or adhesive layer.
Disadvantageously, only substrates of limited dimensions can be sputtered in this process since the polycarbonate is birefringent after a certain substrate size so that information on video or audio-CD-disks, for example, can no longer be sensed fault-free.
A particular problem arises when coating plastics of the group of polymethylmethacrylates, which, for example, are commercially available as "Plexiglass." Here, due to the high sensitivity of this material to ultraviolet radiation, the sputtered layer exhibits a low adhesiveness. The plasma, as a matter of fact, when sputtering with noble gases like neon, argon, krypton and xenon, does predominantly emit ultraviolet radiation in a wavelength range between 70 and 125 nm.